1. Field of the Invention
The present invention relates to a nitride-based semiconductor light-emitting device and a method of fabricating the same, and more particularly, it relates to a nitride-based semiconductor light-emitting device comprising a current blocking layer and a method of fabricating the same.
2. Description of the Background Art
A nitride-based semiconductor laser device, which is one of nitride-based semiconductor light-emitting devices, is expected as an advanced light source for a large capacity disk, and subjected to active development. A well-known general nitride-based semiconductor laser device has current blocking layers consisting of a material having reverse conductivity to a nitride-based semiconductor layer forming a ridge portion on side portions of the ridge portion serving as a current path portion. This nitride-based semiconductor laser device is disclosed in Japanese Patent Laying-Open No. 10-321962 (1998), for example.
FIG. 69 is a sectional view showing an exemplary structure of a conventional nitride-based semiconductor laser device comprising current blocking layers 306 having reverse conductivity to a ridge portion. Referring to FIG. 69, an n-type contact layer 302 of n-type GaN is formed on a sapphire substrate 301 in the conventional nitride-based semiconductor laser device. An n-type cladding layer 303 of n-type AlGaN and an emission layer 304 of InGaN are formed on the n-type contact layer 302. A p-type cladding layer 305 of p-type AlGaN having the ridge portion serving as the current path portion is formed on the emission layer 304. The current blocking layers 306 of n-type AlGaN are formed on the side surfaces of the ridge portion of the p-type cladding layer 305 and on flat portions of the p-type cladding layer 305. A p-type contact layer 307 of p-type GaN is formed on the upper surfaces of the current blocking layers 306 and the upper surface of the ridge portion of the p-type cladding layer 305. A p-side ohmic electrode 308 is formed on the upper surface of the p-type contact layer 307.
Partial regions of the layers from the p-type cladding layer 305 to the n-type contact layer 302 are removed for exposing a surface portion of the n-type contact layer 302. An n-side ohmic electrode 309 is formed on the exposed surface portion of the n-type contact layer 302.
In the conventional nitride-based semiconductor laser device having the aforementioned structure, current flows from the p-side ohmic electrode 308 to the emission layer 304, the n-type cladding layer 303, the n-type contact layer 302 and the n-side ohmic electrode 309 through the p-type contact layer 307 and the p-type cladding layer 305. Thus, a laser beam can be emitted from a region of the emission layer 304 located under the ridge portion forming the current path portion.
In the aforementioned conventional nitride-based semiconductor laser device, the current blocking layers 306 have two functions. First, the current blocking layers 306 are provided on side portions of the current path portion, for feeding the current only to the ridge portion forming the current path portion located substantially at the center of the device. Further, the current blocking layers 306 are prepared from a material having a refractive index different from that of the p-type cladding layer 305, for confining transverse light in the emission layer 304 through the difference between the refractive indices.
In order to strengthen the light confinement in the emission layer 304 in this case, the difference between the refractive indices of the p-type cladding layer 305 located on the emission layer 304 and the current blocking layers 306 must be increased. In order to increase the difference between the refractive indices, the Al composition of the current blocking layers 306 consisting of n-type AlGaN may be increased. In other words, the transverse light can be confined in the emission layer 304 by increasing the Al composition of the n-type AlGaN forming the current blocking layers 306 as compared with the Al composition of the p-type AlGaN forming the p-type cladding layer 305. The nitride-based semiconductor laser device having such a structure is generally referred to as a real refractive index guided laser.
In order to confine the transverse light, the current blocking layers 306 may alternatively be prepared from a material having a smaller band gap than the emission layer 304. When an emission layer and current blocking layers 306 are made of InGaN and the In composition of the InGaN forming the current blocking layers is increased as compared with that of the InGaN forming the emission layer, for example, the current blocking layers can absorb part of light generated in the emission layer. Thus, transverse light can be confined. The nitride-based semiconductor laser device having such a structure is referred to as a complex refractive index guided laser.
In the aforementioned conventional real refractive index guided laser, the current blocking layers 306 of n-type AlGaN are different in Al composition from the p-type cladding layer 305 of p-type AlGaN, and hence the lattice constant of the current blocking layers 306 is different from that of the p-type cladding layer 305. When the Al composition of the current blocking layers 306 consisting of n-type AlGaN is increased, therefore, strain is applied to the current blocking layers 306 to disadvantageously easily cause cracks or crystal defects such as dislocations in the current blocking layers 306. Consequently, it is difficult to form the current blocking layers 306 with a large thickness, leading to difficulty in stabilization of transverse light confinement.
In the aforementioned conventional complex refractive index guided laser, the current blocking layers of InGaN are prepared from the material different from that of the p-type cladding layer consisting of AlGaN, and hence the lattice constant of the current blocking layers is different from that of the p-type cladding layer. When the In composition of the current blocking layers consisting of InGaN is increased, therefore, strain is applied to the current blocking layers to easily cause lattice defects in the current blocking layers. Also in this case, it is difficult to form the current blocking layers with a large thickness, leading to difficulty in stabilization of transverse light confinement.
In the aforementioned nitride-based semiconductor laser device, the current blocking layers 306 are formed above the n-type contact layer 302 of GaN formed in a large thickness. In this case, strain is disadvantageously applied to the current blocking layers 306 due to the difference between the lattice constants of the current blocking layers 306 and the n-type contact layer 302 of GaN having a large thickness.
In the conventional nitride-based semiconductor laser device, further, the ridge portion consisting of the p-type cladding layer 305 is etched by dry etching or the like for thereafter crystal-growing the current blocking layers 306. In this case, the p-type cladding layer 305 consisting of AlGaN is active and hence a surface portion of the p-type cladding layer 305 exposed by etching is easily contaminated with C or O. Therefore, this contaminant infiltrates into the interfaces between the p-type cladding layer 305 and the current blocking layers 306, to disadvantageously cause crystal defects such as dislocations in the current blocking layers 306.
In the aforementioned conventional real refractive index guided laser, the current blocking layers 306 are formed above a substrate or a thick nitride-based semiconductor layer formed on a substrate. When GaN is employed as the material for the substrate or the thick nitride-based semiconductor layer formed on the substrate and the Al composition of the current blocking layers 306 consisting of AlGaN is increased in this case, strain is applied to the current blocking layers 306 since the lattice constant of AlGaN forming the current blocking layers 306 is smaller than the lattice constant of GaN. Therefore, cracks or lattice defects are easily caused on the current blocking layers 306. Consequently, it is difficult to thickly form the current blocking layers 306, disadvantageously leading to difficulty in stabilization of transverse light confinement.
Also in the aforementioned conventional complex refractive index guided laser, the current blocking layers are formed above a substrate or a thick nitride-based semiconductor layer formed on a substrate, similarly to the conventional real refractive index guided laser. When GaN is employed as the material for the substrate or the thick nitride-based semiconductor layer formed on the substrate and the In composition of the current blocking layers consisting of InGaN is increased in this case, strain is applied to the current blocking layers since the lattice constant of InGaN forming the current blocking layers is larger than the lattice constant of GaN. Therefore, cracks or lattice defects are easily caused on the current blocking layers. Also in this case, it is difficult to thickly form the current blocking layers, disadvantageously leading to difficulty in stabilization of transverse light confinement.